1. Field of the Invention
The present invention relates to an echo prevention circuit, a digital signal processing circuit, a method of setting filter coefficients of the echo prevention circuit, a method of setting filter coefficients of the digital signal processing circuit, a recording medium storing a program for setting filter coefficients of the echo prevention circuit, and a recording medium storing a program for setting filter coefficients of the digital signal processing circuit.
2. Description of the Related Art
In these years, among communication apparatuses such as cellular telephones and hand-free telephones to which, e.g., an earphone microphone is connectable are ones having incorporated therein an echo prevention circuit for preventing echoes that occur due to acoustic coupling from a speaker around to a microphone, electrical reflection in circuits, and the like.
FIG. 48 is a diagram showing, e.g., a two-to-four line converter having an echo prevention circuit incorporated therein. An input signal from a microphone 104 is output via an amplifier 101 to a telephone line, and transmitted through the telephone line to a communication partner. Thus, voice can be transmitted. However, the input signal that is transmitted via the amplifier 101 may be output to not only a telephone line but also to a speaker 106 via an amplifier 105 due to electrical reflection or the like in the two-to-four line converter as indicated by the dashed line in FIG. 48. As a result, an echo of the input signal is generated from the speaker 106. Accordingly, to cancel such an echo, an echo prevention circuit is provided in which the input signal from the microphone 104 forks in front of the amplifier 101 to an inverting amplifier 107, which inverts and amplifies the input signal. A signal output from the inverting amplifier 107 is adjusted in gain and phase by a gain phase adjusting circuit (G/P) 108. A signal that is opposite in phase to and has the same amplitude level as the input signal being transmitted as indicated by the dashed line in FIG. 48 is generated. As a result, the input signal transmitted as indicated by the dashed line in FIG. 48 and the signal transmitted as indicated by the solid line in FIG. 48 cancel out thereby preventing an echo. (Refer to Japanese Patent No. 3293029.)
However, the amount of echo cancellation by the above echo prevention circuit is about −30 dB. This is because, although for the echo prevention circuit to cancel an echo highly accurately, circuit constants of its circuit elements need to be set highly accurately, in reality it is not easy to set the circuit constants of the circuit elements highly accurately. Thus, there is the problem that if the circuit constants deviate from desired values, the amount of echo cancellation decreases. Furthermore, there is the problem that due to errors in adjusting gain and phase by the gain phase adjusting circuit 108, it is difficult to generate a signal to cancel out the input signal transmitted as indicated by the dashed line in FIG. 48. As a result, the amount of echo cancellation by the above echo prevention circuit is about −30 dB.
Moreover, even if a voice signal over the telephone line is weak, the amplifier 105 of FIG. 48 amplifies the voice signal to have a given amplitude level so as to make voice from the weak voice signal audible. However, since the amplifier 105 is provided, there is the problem that echoes due to the amplified output of the amplifier 105 cannot be prevented. Accordingly, there is a demand for an echo prevention circuit to effectively prevent such echoes.